The field of the invention is in the gas laser art.
Transverse-discharge gas lasers with cold cathodes are well known. Generally, they have the limitation that the cathode fall cannot be reduced below approximately 80 to 100 volts; even when cathode materials with the highest yield of electrons liberated by impacting ions are used. This limitation causes the following disadvantages. The energy losses in these tubes are much higher than those dictated by optimum energy distribution of the electrons injected into the plasma (for maintaining the plasma and for excitation); the cathode sputtering is much more severe than it would be under optimum conditions; and the energy distribution of the electrons injected into the plasma cannot be optimized with respect to laser action. Typical examples of prior art cold-cathode devices are exemplified by U.S. Pat. Nos. 3,396,301 to patentees Kubayashi et al., and 3,787,781 to patentees Medicus et al.
Thermionic cathode transverse discharge lasers are known as disclosed by U.S. Pat. No. 3,719,899 to patentee Breaux. Thermionic hollow cathodes for lamps are disclosed in U.S. Pat. No. 3,558,964 to patentee White. The typical electrode configurations in the prior art slotted-hollow-cathode, FIG. 8, and plasma slab lasers require comparatively large cathode surface areas and, consequently, require large and bulky heaters when thermionic cathodes are used. To obtain uniform cathode temperatures and uniform emission currents is difficult due to the prevalency of concentrations of the discharge to localized areas, inhomogeneous cathode emission, and cathode spot formations.